.
-
..,
/
ICES 1989
PAPER
C.M.1989/E:32
THE POTENTIAL USE OF SEDIMENTS AND SUSPENDED MATIER AS A MONITORING
TOOL FOR POLLUTION IN ESTUARIES AND COASTAL REGIONS.
by
•
ABSTRACT
The use of ~tdiments and suspended matter as a monitoring tool for pollution in estuaries and
coastal regions is a matter of preoccupation of various national and international authorities. The
sediments have obviously the advantage to integrate on a long time scale (depending on the
sedimentation rate) modifications of the quality of the water column. Since many contaminants,
organic and inorganic, are preferentially transferred from the aqueous phase to the suspended solid
phase in the aquatic environment, sediments become thus rapidly the final depository for these
contaminants.
Sedimentation in the estuarine and coastal regions is a very complicated process influenced by the
local hydrodynamical conditions. Deposited sediments may be further affected by physical and
biological perturbations or early diagenetic reactions. The evaluation of background levels of
contaminants prior to the disturbances due to human activities as a reference level constitutes also
a delicate problem.
.
•
Many of these problems can be solved by the use of adequate normalizing techniques. Our contribution is an attempt to use the analysis of the fraction less than 20 ~m or to normalize the concentrations of trace metals with respect to Al in the case of suspended matter and sediments collected
in the Scheldt in order to evaluate the degree of contamination of the estuary. Various original
methods of collection of suspended matter, of size fractionation and of chemical analysis have been
therefore developed and tested.
Furthermore, a method based on the introduction of the solid sampie previously dispersed in a
viscous medium (slurry sampling) has been developed and applied to the analysis of suspended
matter collected in the Scheldt estuary.
THE POTENTIAL USEOF SEDIMENTS AND SUSPENDED MATIER AS A MONITORING
TOOL FOR POLLUTION IN ESTUARIES AND COASTAL REGIONS
P. REGi'm:R(l), R. WOLLAST(l) and M. I10ENIG (2)
(l) Laboratory of Chemical Oceanography, University of ßrussels, ßelgium
(2) Institute for Chemical Research, Ministry of Agriculture, Tervuren. ßelgium
•
AßSTRACT
The use of sediments and suspended matter as a monitoring tool for pollution in estuaries and
coastal regions is a matter of preoccupation of various national and intermitional authorities.
The sediments have obviousiy the advantage to integrate on a long time scale (depending on the
sedimentation rate) modifications of the quality of the water column. Since many contaminants,
organic rind inorganic, are preferentially transferred from the aqueous phase to the suspended solid
in thc aquatic environment, sediments become thus rapidly the final depository for these contaminants.
,
Sedimentation in the estuarine and coastal regions is a very complicated process influenced by
the local hydrodynamical conditions. Deposited sediments may be further affected by physical and
biological perturbations or early diagenetic reactions. The evahiation of background levels of
contaminants prior to the disturbances due to human activities as a reference level constitutes
also a delicate problem.
Many of these problems can be solved by the use of adequate normalizing techniques. Our
contribution is an attempt to use the analysis of the, fraction less than 20 pm or to normalize
the concentrations of trace metals with respect to Al in the case of suspended matter and
sediments collected in, the Scheldt in order evaluutc the degree of contamination of the estuary.
Various original methods of collection of suspended matter, of size fractionation and of chemical
analysis have been therefore developed und tested.
,
,
'.
Furthermore, a method based on, the introduction of the solid sampie previollsly dispersed in u
viscous medium (slurry sampling) has been developed arid applied to the analysis of suspended
matter collected in the Scheldt Estuary.
INTRODUCTION
The use of sediments and suspended matter as a monitoring tool in relation to pollution has
•
been a subject of concern and discussion at various national and interriational levels since several
years (lCES, 1981).
In general, most contaminants show high affinity to particulate matter and preferentially to the
fine grained fraction of the suspended solids. These particles tend then to be accurriulated' by
sedimentation in areas of low hydrodynamic em;rgy. orie of the main advantages of sediments is
,that they te nd to concentrate many containinants and they integrale, over rather long time
scales, the usually fluctuating input of many industrial potlutants such as toxie metais. However,
sediments are submitted to various physieal, chemical arid biological processes which renders their
use as a monitoring tool often difficult, espeeially in estuaries and in the coastal zones, charaeterized by strong hydrodynamical and physico-ehemical gradients (Larsen et al., 1988, Eisma et
al., 1989).
It is therefore essential to take into aecount the physical and chemical eharaeteristics of the
sediments arid to understand their distribution and origin in order to eväiuate the extent of their
possible anthropogenie contamination.
Many of these problems ean be slrilplirJed if an adequate normalization procedure is used in
order to characterize the origln of the sediments and their ability to aecumulate pollutants of
interest (Loring, 1988). T;le most commonly normalization factors that have been suggested' are
grain size and the use of elements characteristic of the fine fraction of sediments Iike AI, Fe and
organic matter (ICES, 1988).
We will discuss here briefly the application of these normalization techniques in the case of the
sedimens of the Scheldt.
In a second part of this report, we will describe a method that we have developed to analyze
trace metals in small amounts of suspended matter, collected by filtration in the estuary. Due to
the great affinity of trace elements to the particulate phase it is often pdmordial to characterize
the composition of the susperided matter in a river if a realistic and complete assessment of the
•
potential pollution is to be perforrned (Wollast, 1982). The distribution of elements between the
dissolved and the particulate phase is also an essential factor for the understanding of the
geoeheinical behaviour of the element considered. There are however only a few systematie
stlldies of the composition of the particulate. matters in aquatic systems because the sampling and
the analytical procedures are often tedious or require sophisticated equipments tike the neutron
activation analysis. The method presented here is based on the direct analysis of asolid sampie
by atomie absorption.
.
,
,
UTILISATION OF NORMALIZATION FACTORS FOR THE SEDIMENTS OF TUE SCHELDT
A detaiied distribution of major arid trace elements as a function of particle size has been
performed on two typical mud sediments of the Scheldt estuary collected in the harbour of
Antwerp.
The sediments were ultrasonically dispersed in a solution of sOdiurri polyphosphilte in distilIed
water and separated by elutriation in 6 grariulometric fractions. The granuloinetric fractions were
01,
•
further filtered and dried.
Two digestion ilH~thods were used :
- a total dissolution using a mixture of HF + HCI04 in a platinum erueible ;
- a strong aeid digestion using reflux boiling of
a mixturc
of concentrated
Iici ...
HNOj (3+1)
du ring 2 hours.
The major elements (AI, Ca, Mg, K, Na, Fe, Mn) were measured by induetively eoupled plasma
emission spectrometry and the mirior elements (Zn, Cu, Pb, Cr, and Ni)by flame atomic absorption
spectrometry or graphite furnace atomic absorption spectrometry (Co, Cd).
Thc results for thc distribution of Pb, Cr and AI in the various granulometdc fractions after
total decomposition of tlte sampIes are shown
as selected exampies in iigure 1.
The results are very similar for the two sainples ::irid demonstrate very clearty the increase of
concentration of trace elements in the fine fractions. SimlJar results have beIm obtained for all
the traee metals investigated. These results indieate that most of thc traee metals are present in
the fraction smaller than 20 J.lm. The relative amount of the fraction less than 20 J.lm may thus
constitute a good normalizing parameter in the case of the Scheldt sediments.
It can also be seen in figure 1 that there is a constant increase of the AI content with
decreasing particle size. The AI content of the sediment represents thus a good indication of the
amount of fine material present in the sampie.
AI. %
12 r - - - - - - - - - - - - - - - - - - - ,
CJ
Sadl
~ Sad2
•
>50
>32
>16
'8
>4
<4
Fraction, IJm
P_b~1.:-\-l~g/~g~
.....,
180.,...
er, IJg/g
300 ,...-.;...;.....::......::..------------.,
250
200
150
100
•
'50
>32
>16
'8
Fraction, \-lm
Fig. 1.
>4
<4
>50
>32
>16
'8
'4
<4
Fraction, I-lm
Granulometric distribution of Pb. Cr and Al in the case of two typical muddy sediments of the Scheldt estuary.
It is of course much easier to measure the Al content of a sediment than to perform the tedious
size fractionation necessary to recover the Iess than 20 J.lm fraction. We have also calculated the
regression coefficient between the concentration of all the trace elements measured and the
aluminium content of the sediments. The results of these calculations are represented in table I.
Since there is also an excellent correlation between Al and Fe. iron can also be used as a good
normalizing factor in the case of Scheldt sediments. It is not the case for Mn which is strongly
affected by remobilization processes due do the very reducing condition existing in the Scheldt
estuary (Wollast. 1982).
Manganese exhibits however a significant positive correlation (r
= 0.72)
with Ca which is
representative of the carbonate fraction of the sediments.
The results obtained with the strong acid extraction are inuch less satisfactory due to partial
mobilization of Al and of many trace elements even in the fraction less than 4 J-lm;
The normalization technique with respect to size fraction und Al content has been applied to
compare the results obtained for a few typical sediments und for suspended matter of the Scheldt
estuary (Fig. 2).
TADLE 1.
•
Correlation coefficients of elements in the various size fractions of two sediments of
the Scheldt.
AI
Zn
Cu
Cd
Pb
Cr
Ni
Co
Fe
Mn
0.82
0.83
0.73
0.93
0.99
0.79
0.88
0.98
0.29
Two sediments were collected in the marine part of the estuary dominated by sandy deposits
mainly of marine origin which were transported upward the estuary by bottom currents generated
by the salinity gradient.
Three sampies of sediments were collected in the muddy area of the estuary Iocated between km
50 and 100 where intensive shoaling occurs. Most of the fine material transported by the river
isfloculated and deposited in that area. This. fine fraction is however mixed and partially diluted
with coarse sands of marine origin transported upwards by bottom currents. At these three
stations we have also collected continuously the suspended matter during four months by taking
one instantaneous sampie of 2 liters per tide. The suspended matter accumulated during one inonth
period was then carefully homogenized. The composition of the suspended was found to be
remarkably constant at each station.
•
We ~ave also collected in the same area a sampIe of a quarternary clay deposit (Rupelian) which
is presently being eroded by the river.
The .granulometric fraction less than 20 J-lm was extracted by elutriation from bOlh sediments
and suspended matter. :md was analyzed for major and minor elements. The results are presented
in tables 2 and 3.
The analyses of the suspended matter normalized with respect to the size fraction are very
consistant and normalization appears to be very satisfactory. Since the Al content is rather
constant in the less than 20 J-lm fraction both criteria reflect weIl the accumulation of the trace
elements in the fine fraction. It is remarkable to see that for all the elements considered there
is a systematic decrease of the concentration of trace elements with decreasing distance to the
sea. There may be two explanations for this observation: the fine fraction rieh in trace elements
is preferentially removed by sedimentation in the lower salinity area of the estuary and/or the
highly contaminated suspended maUer of fresh water origin is' progressively diluied by less
coi1taminated and more marine suspensions.
•
ojl---"""!I10km
Fig. 2.
The Seheldt estuary ; loeation of the suspended matter sampies (0 : Ooel,
L : Linkeraever and H : Hemiksem)
Normallzatlon wlth
respact to AI
( 1)
D.
AI
P
Zn
Cu
Cd
•
Pb
Cr
Ni
Co
6.48
4910
834
144
12
230
200
67
19
(2)
L.
6.90
5880
1000
178
13
276
235
97
24
(3)
H.
6.75
6590
1130
200
14
328
255
100
24
D.
L.
H.
75.7
12.9
2.22
0.18
3.55
3.09
1.03
0.29
85.2
14.5
2.58
0.19
4.00
3.41
1.40
0.35
97.6
16.7
2.96
0.21
4.86
3.78
1.48
0.38
( 1) Doal, km 61.5
(2) Linkaroever, km 78.0
(3) Hemiksem. km 87.5
TADLE 2
Composition of the less than 20 J.lm fraetion of suspended matter eolleeted in the
Seheldt (in ppm exeept for Al in %).
The use of normalization teehniques appears more powerful in the ease of sediments beeause of
the mueh larger speetrum of their particle size. The distribution of contaminants in the total
sediment gives only very braad information and selective dissolution techniques obscure the distribution pattern rather than enlight it (W.ollast et al., 1985).
The use of the less than 20 Jlm fraction allows not only to distinguish clearly the considerable
enrichment of some trace elements in the fine fraction of the sediments but also to compare with
an uncontaminated sediment (sed6) to· evaluate the level of contamination of the area. The
number of sampIes considered here is not sufficient to discuss in detail the sources and sinks of
contaminants in the estuary. Dut it gives nevertheless some general ideas about the degree of
pollution in the upper part of the estuary.
With no doubt such a preliminary investigation can lead to the development of a very powerful
monitoring strategy based on the use of sediments.
Dislance km
AI
P
•
Zn
Cu
Cd
Pb
Cr
NI
Co
sedl
sed2
sed3
sed4
sod5
sed6
4
45
57.5
76
87.5
82.8
5.76
1923
250
35
<1
87
92
33
12
7.02
1481
142
28
<1
32
85
48
14
6.74
5157
682
157
10
200
210
65
20
6.39
5143
909
172
14
251
200
68
19
9.70
4815
1798
1981
NA
208
124
76
12
10.44
1517
141
93
<1
29
117
60
12
77
80
14.2
2.69
0.22
3.93
3.13
1.06
0.30
50
18.5
20.4
NA
2.14
1.28
0.78
0.12
15
1.4
0.89
<.01
0.28
1.12
0.57
0.13
Normo.llzntion wilh rospecl 10 AI
P
Zn
Cu
Cd
Pb
Cr
NI
Co
TADLE 3
•
33
4.3
0.61
<.01
1.51
1.60
0.57
0.21
21
2.0
0.40
<'01
0.46
1.21
0.68
0.20
10.1
2.33
0.15
2.97
3.12
0.96
0.30
Composition of the less than 20 Jlm fraction of the sediments collected in the Scheldt
(in ppm except for AI in %)
ANALYSIS OF TRACE METALS IN SUSPENDED MATTER COLLECTED BY FILTRATION
We have recently developed an analytical procedure especially adapted for the determination of
trace metals in small quantities of solids, generally 1 to 10 mg. This method is based on the
direct analysis of solid sampIes using electrothermal atomic absorption where the particles are first
dispersed in a viscous medium ("slurry sampling") (Hoenig et al., 1986).
This technique is similar to the solution analysis and can be easily automated for routine use.
It allows sequential multi-elements analyses of Cd, Cu, Co, Cr, Ni and Pb on the same sampIe
(Hoenig et al., 1989). The performance of this method has been investigated by calibrating four
international soil and sediment standards (Table 4). Different tests of the reproducibility of the
calibration curve and of the analysis itself have been conducted in the case of the suspended
matter collected in the Scheldt. Figure 3 shows the stability of the absorbance signals for Co, Ni
and Cr, indicating the stability of the slurried sam pIe.
•
absorbance Co.Cr
absorbance Ni
0.4 r - - - - - - - - - - - - - - - - - - - - - - - - , 0 . 1 5
0.3
0.1
0.2
Q.1 '--
o
'--
'--_ _---'L.-_ _---'
10
20
30
40
----'
50
---'
0.05
60
time. mlnutes
..
~
-4-Cr
Fig.3.
-e-Co
-NI
Stability of the slurries with time: successive absorbance signals of Ni. Cr. and Co
(suspended matter sampIe of the Scheldt estuary).
RESULTS AND DISCUSSION
Suspended matter has· been sampled at various locations of the Scheldt estuary on 29/03/88 and
from I to 3/11/88. The particles were recovered by filtration through 0.45 J.lm membrane filters.
The ability of this method to record rapidly a longitudinal profile of the composition of the
suspended matter is demonstrated in figure 4 where the distribution of the concentrations of Cu is
presented for March and November 88. Particulate copper behaves obviously not like a conservative component in the upper estuary.
In March. characterized by a high river discharge. the concentration of copper in the suspended
•
matter is relatively low in the fresh water part which may be due to dilution of the contaminants.
The maximum observed in the upper estuary may simply reflect departures from steady-state
conditions due to a rapid increase of the fresh water discharge also characterized by a tower level
of contamiriation of the suspended matter.
At higher salinities. the mixing curves presented in figure 4 indicate mainly a deerease of the
concentration of copper in the suspended matter. Similar trends are observed for the other traee
metals (Fig. 5).
This deerease may be due either to desorption and solubilization of partieulate metals when the
salinity inereases or to the dilution of highly contaminated partieulate matter of continental origin
by marine suspended matter with rather low levels of traee meta) contents.
A simple dilution process of suspended matter of terrestrial and marine onglO should give a
straight Hne in figures 4 and 5. whieh seems to be the ease for most metals. Positive or negative
deviations from the straight Hne indicate either uptake or release of trace metals by the particulate phase. Alternatively loeal input of particulate metals or removal by preferential sedimentation
of contaminated particles may lead to the same effect. Ideally concentration in both dissolved and
particulate phases should be performed along longitudinal profiles.
Until now, the particulate phase has been very much neglected, mainly because of the difficulties encountered in sampling large representative quantities of suspended matter required for the
chemical analysis.
Element
Cu
..
~
•
SRM
SOIL-7 I)
ESTUAR. SED. 2 )
LAKE SED. 1)
MARINE SED. I)
Recommended
value, ppm
11
18+3
30+5
72.2
Found
value, ppm
% RSD
(n = 4)
13.5
20.3
32.3
76.3
3.9
5.5
6.8
2.8
1.14
0.35
0.28
9.0
1.5
5.0
5.0
2.1
SOIL-7 I)
ESTUAR. SED.2)
LAKE SED. 1)
MARINE SED. I)
(1.3)
0.36+0.07
0.26+0.05
Pb
SOIL-7 1)
ESTUAR. SED. 2)
LAKE SED. I)
MARINE SED. I)
60
28.2+1.8
37.7+7.4
120
62.1
26.8
34.1
120-
1.8
8.6
2.9
5.4
Cr
SOIL-7 l)
ESTUAR. SED. 2 )
LAKE SED. I)
MARINE SED. I)
60
76+3
104+9
149
62.1
72.5
95.1
135
4.7
3.2
21.1
4.3
Co
SOIL-7 I)
ESTUAR. SED. 2 )
LAKE SED. 1)
MARINE SED. 1)
8.9
10.5+1.3
19.8+1.5
12.1
Q.O
10.7
22.3
12.8
1.0
5.5
18.1
8.3
SOIL-7 1)
ESTUAR. SED. 2 )
LAKE SED. 1)
MARINE SED. 1)
(26)
32.0+3.0
44.9+8.0
31
29.5
33.5
40.3
29.3
6.2
10.1
17.5
3.0
Cd
Ni
II
l): International Atomic Energy Agency (IAEA); SRMs: SOIL-7, SL-I and SD-Nlj2
2) : National Bureau of Standards (NBS) ; SRM 1646.
TABLE 4
Analytical results on standard reference materials obtained by graphite furnace atomic
absorption spectrometry using "slurry" sampling technique (Hoenig et aL, 1989).
..
C
_:::...op!:!p:....:e:.:...r.:..-,~l-1g~/-:::g~
150r-
C__
o.:-pp:.-e..:..-r...:..,...:..1-1...=:9_1g.::::.....
-.
250,
---,
November 1988
March 1988
120
90
60
100
30
50
o L - _ - - l ._ _-1-_ _-'--_ _'--_--I.._ _....l
o L-_ _...l-_ _--'-_ _----''--_ _-'--_ _--'
o
5
10
15
20
25
o
10
5
Salini ty, 9/1
Fig.4.
•
15
20
25
30
Salinity, 9/1
Longitudinal profile of the concentration of copper in the suspended matter collected
in the Scheldt in March and November 1988 .
C_~a~d_m_iu_m-..:,....:.I-1....::g::..../..:::9:...--
---,
12.
__,
200
*
9
C
;:.~h::....:ro:..:.m..:...:.:..::iu::....:m~,~I-1::::g~/...=:g:__
250
*
150
6
*
100
*
3
*
50
0'-----'----1..---1----'-----'-----'
o
10
5
15
20
25
oL-_--i_ _- l -_ _-'--_ _L . - _ - - l ._ _...J
o
30
5
10
Salinity, g/I
•
250
Lead, I-1g/g
50
200
•
25
20
25
30
Nickel. 1-19/g
•
30
100
20
50
10
0
5
10
15
Salinity, g/I
Fig.5.
20
40
150
0
15
Salinity, g/I
20
25
30
0
0
5
10
15
Salinity, g/I
Longitudinal profile of the concentration of Cd, Cr, Pb and Ni in the suspended
matter collected in the Scheldt estuary (November 1988).
30
,--•
CONCLUSIONS
Our recent work on the Scheldt estuary has been mainly devoted to the development of sampling
strategies rind analytieal proeedures in. order to use sediments and partieulate matter as monltoring
toois in pollution studies.
We have shown that normalization of the trace metal content in sediments and suspended matter
can be carrhid out either by anaiysing the fraction less than 20
Ilm or. by expressing the
results
of the analysis of traee metals with respeet to the AI eontent of the solid phase. Small but
consistent f1uctuations in composition can be detected when such normalization is performed and
ean give interesting indieations coneerning the origin behaviour and fate of eontaminants.
We have on the other hand developed a rapid method of analysis of partleuiate matter requiring
only a few mg of sampIe. This method allows to eonsider the use of tite eompositicin of suspended
matter as a water quality lndicutor. It would be of major interest to deveio p more systematicaÜy
•
simultuneous measurement
>
01' both
the particulate und dissolved fraction of trace elements in order
to gain a beiter understanding of their behaviour in the aquatie systems.
ACKNOWLEDGEMENTS
.
. '
This work was partially supported by the EEC - Radiation Proteetion Programme, eontraet rir.
ßI6-Dl91-NL.
\\re
are grateful to the Management Unit North-sea and Scheldt estuary for its
logical assistance. We thank the Officers and the Crew of the R.V. Deigica and R.V. Luctor for
their cooperation during the eruises on the Scheldt.
REFERENCES
•
Eisma, ri.~ Derger, G.N., Chen \Vei-Yue and Shen Jian, 1989.. Pb-21O as a tracer for sediment
transport and deposition in the Dutch-German Waddensea. In : Proceedings. KNGMG
Symposium "Coastal Lowlands, Geology and Geotechnology", Kluwer Academie Publishers,
.
.
.
'.
. •.
.
. Dordrecht, pp. 237-253.
1I0enig, M. and Van Hoeyweghen, P., 1986. Alternative to solid sampling for trace metal determination by platform electrothermal atomie absorption spectrometry : direct dispensirig of
.
powdered sampIes suspendedin liquid medium. Anal. Chern. 58: 2614-2617.
1I0enig, M.; Regnier, P. and Wollast, R., 1989. Automated trace metals analyses of slurfied solid
sampIes by GFAAS with application to sediments rind susplmded matter collected in natural
.waters. J. Anal. Atom. Spectrcim., in press. .
ICES, 1981. Report on the first meeting of the Working Group on marine sediments in' relation to
pollution. ICES Publ., Copenhagen, 20 pp.
.
ICES, 1988. Report of the 75th stritutory meeting of the Working Group ori Marine Sediments in
.
relation to pollution. ICES PubI., Copenhagen, 86 pp. ,',
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,..
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Larsen, D., Madsen, P.P. and Jensen, A., 1988. Sensitivity of sediments in pollution monitoring. In :
Report of the 75th statutory meeting of the WGMS in relation to pollution, annex ICES
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Publ.; Copenhagen, pp. 6 1 - 6 8 . , .
.
Loring, 0.11., .1988. Normalization of heavy metal data. In : Report of the 75th statutory meeting
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of the WGMS in relation to pollution, annex 2. ICES Publ., Copenhagen, pp. 19-42. . .
Wollast, R., 1982. Methodology of research in micropollutants - heav}i rnetals. Wat. Sei. Tech.,
,
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..'
.. ' ,
..
Wollast, R., Devos, G. and 1I0enig, M., 1985. Distribution of heavy metals in the sediments of the
Scheldt Estuary. In': Van Grieken R. and Wollast R. (Editors). Proceedings Progress in
Delgian Oceanographie Research. Royal Academy of Seiences, Drussels, pp. 147-159.